1. Field of the Invention
The present invention relates to a thin film forming apparatus and more specifically a thin film forming method suitable for forming electrodes and protective films for semiconductor devices, electrodes and protective films for liquid crystal apparatus, protective films for photomagnetic recording media, and antireflective films and reflection enhancing films for optical articles.
2. Related Background Art
Sputtering is known as one of thin film forming methods.
In a conventional reactive sputtering, a gas mixture of a sputtering gas and a reaction gas is introduced into a reaction chamber, and a compound target or a metal target is sputtered to form a thin film of a metal compound by a chemical reaction between atoms constituting a target and a reaction gas. A depositing rate is generally low for an insulating compound target, though electric discharge can be caused by supplying a high-frequency electric power such as of RF or microwave. For a metal target, it is possible to cause electric discharge by supplying a DC voltage, thereby enhancing a depositing rate.
Even when a target is made of a metal, a reaction gas reacts with the metal target on a surface thereof, thereby forming a metallic compound on the surface of the target. Since the sputtering yield with respect to a metallic compound is about 10% of the sputtering yield with respect to a metal, the deposition rate is small in reactive sputtering. Even in case of a compound target, since the thin film formed from the compound has a high content of metal atoms, it is necessary to add a reaction gas for obtaining a film which has a composition close to a stoichiometric ratio of the compound. When the flow rate of a reaction gas decreases in the case of a metal target, a formed thin film of a metallic compound has a large content of metal atoms, whereby the formed film cannot satisfy a stoichiometric ratio thereof and is inferior in its optical characteristics (refractive index, transmittance, etc.) and so on.
Accordingly, there have been proposed several attempts made to solve such a technical problem.
FIG. 6 is a schematic view of a reactive sputtering apparatus disclosed by Japanese Patent Application Laid-Open No. 62-56570. A reference numeral 1 indicates a target, 2 a substrate, 3 a tube for supplying argon (Ar) to be used as a sputtering gas, 4 a tube for supplying oxygen (O.sub.2) to be used as a reaction gas, 9 a reaction chamber, 12 a target holder and 7 a substrate holder.
According to the application mentioned above, it is reported that use of the apparatus shown in FIG. 6 enhances a sputtering rate and improves characteristics of an oxide since it introduces the sputtering gas and the reaction gas separately, thereby causing sputtering preferentially in the vicinity of the target and allowing an oxidizing reaction to take place preferentially in the vicinity of the substrate.
In actuality, however, the sputtering gas is mixed with the reaction gas between the target and the substrate, thereby producing plasma of a mixture of both the gases. When a thin film is to be formed on the substrate of a large area in particular, a discharge region is large between the substrate and the target. Therefore, improvement of a film quality and increase of a sputtering rate cannot be expected.
On the other hand, FIG. 7 shows a schematic view of a reactive sputtering apparatus disclosed by Japanese Patent Application Laid-Open No. 6-41733. A reference numeral 1 indicates a target, 2 a substrate, 3 a tube for supplying argon to be used as a sputtering gas, 4 a tube for supplying oxygen to be used as a reaction gas, 9 a reaction chamber, 12 a target holder, 7 a substrate holder, 8 a power source, 13 a differential pressure plate, 14 a high-frequency power source, 15 an exhaust pump, 16 magnets and 17 a tube for circulating a refrigerant.
In this apparatus, an attempt is made to separate a sputtering gas from a reaction gas by providing an exhaust opening communicating with a vacuum pump in an upper section of the reaction chamber 9 and by generating a pressure difference between the upper section of the reaction chamber and a lower section of the reaction chamber by utilizing the differential pressure plate 13.
Japanese Patent Application Laid-Open No. 7-335553 discloses a reactive sputtering apparatus which has been proposed to achieve an object different from that of the above-mentioned apparatus. In this apparatus, a collimator having a high aspect ratio is provided between a target and a substrate so as to fill a contact hole of a semiconductor device, thereby narrowing an angle which allows sputtered component atoms of the target to be incident onto a surface of the substrate.
However, the apparatus shown in FIG. 7 enlarges a region of discharge since the differential pressure plate 13 has an opening 13a which is larger than the substrate 2 and a sputtering gas actually flows to the substrate 2 through the opening 13a of the differential pressure plate 13. Accordingly, improvement of a sufficient sputtering rate or film characteristics cannot be as expected. Further, this apparatus requires preliminary excitation of oxygen with a high frequency power source 14, an inside surface of a reaction gas supply tube 4 is sputtered due to the preliminary excitation, thereby resulting in a detrimental effect that a component substance such as iron of a reaction gas supply tube is incorporated into a film to be formed.
Further, this apparatus is apt to allow the substrate to be excessively heated by sputtered particles which jump into the substrate.
Any of the above methods adopted for the attempts cannot be used as a method of forming a minute film at a great rate.
Describing an example which is easy to understand, there results in a phenomenon that a refractive index is lower than a designed value when a film is formed of a transparent material such as an alumina.